Combine the cranberry juice, agave and mint in a medium saucepan over medium heat just until it comes to a boil. Whisk to combine, then cover and steep until the cranberry juice tastes like mint, 3 to 5 minutes. If not sweet enough for your liking, add a bit more syrup. Strain out the mint and pour the liquid into a pitcher.
Stir the rum and lime juice into the pitcher and refrigerate until cold.
To serve, add 3 to 4 mint leaves, a few cranberries and ice to lowball or wine glasses. Pour in the chilled drink, then top off with a splash of sparkling water or ginger ale. Enjoy!